Channel estimate predicted from several previous channel estimates, for pre-equalization

ABSTRACT

A data transmission method includes transmitting an encoded data signal in form of a data stream of data bursts between a transmitter and a receiver, making a decision as a function of at least one data transmission parameter as to whether an interference treatment of the data signal to be transmitted will be performed in the transmitter or in the receiver, performing the interference treatment in terms of at least one code in the transmitter, and performing the interference treatment of the data to be transmitted in terms of at least one further code in the receiver.

BACKGROUND OF THE INVENTION

The present invention relates to a data transmission method, in which anencoded data signal is transmitted in the form of a data stream of databursts between a transmitter and a receiver, and to a corresponding datatransmission apparatus.

Although in principle the present invention can be employed forarbitrary data transmissions, it will be explained along with theproblems it seeks to solve with regard to a cellular CDMA (Code DivisionMultiple Access) data transmission system.

In particular, by means of CDMA, a plurality of data streams can betransmitted simultaneously over a joint frequency band, as known forinstance from K. D. Kammeyer, Nachrichtenübertragung [CommunicationsTransmission], 2nd Edition, in the series entitled Informationstechnik,published by Teubner, Stuttgart, 1996.

In CDMA, a simultaneous transmission of multiple data is effected byencoding the data with codes that are as much as possible uncorrelated,and by superposition of the encoded signals.

In data transmission, among other things, data transmitted in successionor simultaneously interfere with one another; that is, troublesomeinterference typically occurs in the transmission, in particularintersymbol interference (ISI) from multi-path transmission, andmultiple access interference (MAI) by correlated codes.

The following methods are known for interference treatment:

-   -   a rake receiving device, located in the receiver, for treating        the ISI, as known from John G. Proakis: “Digital        Communications”, 3rd Edition, McGraw-Hill, New York, etc., 1995;    -   joint detection (JD) in the receiver for treating ISI and MAI,        as known from A. Klein, G. K. Kaleh and P. W. Baier: Zero        Forcing and Minimum Mean-Square-Error Equalization for Multiuser        Detection in Code-Division Multiple Access Channels, IEEE Trans.        Vehic. Tech., Vol. 45 (1996), 276-287;    -   pre-rake combining in the transmitter for treating ISI, as known        from R. Esmailzadeh and M. Nakagawa: “Pre-Rake Diversity        Combination for Direct Sequence Spread Spectrum Mobile        Communications Systems”, IEICE Trans. Comm., Vol. E76-B (1993),        1008-1015; and    -   a joint preequalization in the transmitter for treating ISI and        MAI.

In this type of data transmission, preequalization in the transmitter isan important interference treatment. Joint preequalization in thetransmitter of data signals to be transmitted makes simple datadetectors possible. In transmission channels that change quickly,however, preequalization in the transmitter leads to higher error ratesthan with interference treatment techniques in the receiver. Forpreequalization, the transmitter must know the pulse response of thetransmission channel to be used. The TDD (Time Division Duplex) methodmakes it possible to achieve this knowledge. Accordingly, the channel isestimated before the data transmission.

The following have proved to be disadvantages of the prior art:

-   -   the rake receiving device does not eliminate MAI;    -   JD is very complicated;    -   joint preequalization is usable only for channels that change        slowly.

SUMMARY OF THE INVENTION

A concept fundamental to the present invention is that a combination ofjoint preequalization in the transmitter and interference treatment inthe receiver is performed. The signals to be transmitted are accordinglyjointly preequalized in such a way that for some of the data to betransmitted, interference can be eliminated in the transmitter, and theinterference of the other data can be treated in the receiver.

This has the particular advantage of making joint preequalization andinterference treatment in the receiver possible in a single system. As aresult, a simple receiver with a channel that changes slowly ispossible, and at the same time, transmission with a channel that changesquickly is possible.

Another concept fundamental to the present invention is that in aninterference treatment in the transmitter, the channel estimate requiredfor the preequalization is predicted from a plurality of prior channelestimates.

This has the particular advantage that the channel estimate bettercorresponds to the channel at the instant of transmission. This improvesthe transmission when the transmission channel is changing quickly.

Another concept fundamental to the present invention is that an estimateof the channel pulse response used by the transmitter is performed bythe receiver, and the data are detected using this channel estimate.

This also has the particular advantage that the transmission when thetransmission channel is changing quickly is improved.

Advantageous refinements of and improvements to the applicable subjectof the invention are found in the dependent claims.

In a preferred refinement, the interference treatment is performed interms of at least one code in the transmitter, and the interferencetreatment of the data to be transmitted is performed in terms of atleast one further code in the receiver.

In a further preferred refinement, the data signal is CMDA-encoded.

In a further preferred refinement, the codes of the data signal to betransmitted are pre-distortion-suppressed jointly, and for some codes,the pulse function is used as a channel estimate.

In a further preferred refinement, the data with a channel estimateperformed as a pulse function are detected in the receiver by a rakereceiving device.

In a further preferred refinement, in an interference treatment in thetransmitter, the channel estimate required for the preequalization ispredicted from a plurality of prior channel estimates.

In a further preferred refinement, the channel estimate to be predictedis calculated from the prior channel estimates by linear extrapolation.

In a further preferred refinement, an estimate of the channel pulseresponse used by the transmitter is performed by the receiver, and thedata are detected using this channel estimate.

In a further preferred refinement, the received signal is filtered inaccordance with the estimated pulse response, and then the data aredetected by means of a rake receiving device.

In a further preferred refinement, the estimated pulse response is usedfor pseudoinverse detection.

BRIEF DESCRIPTION OF THE DRAWINGS

One exemplary embodiment of the invention is shown in the drawing andwill be explain in further detail in the ensuing description.

Shown are:

FIG. 1, an illustration of a transmitter device for explaining a firstembodiment of the invention;

FIG. 2, an illustration of a transmitter device for explaining a secondembodiment of the invention;

FIG. 3, an illustration of a receiving device for explaining a thirdembodiment of the invention; and

FIG. 4, an illustration of an estimating procedure for the channelestimate used for signal transmission, by means of the receiving deviceof FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, the same reference numerals identify components thatare the same or functionally the same.

FIG. 1 illustrates a transmitter device for explaining a firstembodiment of the invention.

In FIG. 1, the reference numerals, letters and characters have thefollowing meanings: 1 is a transmitter; D stands for the data to betransmitted; 10 is a modulator; 20 is a preequalizer; 30 is a switch; 40is a channel estimator; delta stands for pulse functions; 50 is anantenna; and 100 is a TDD radio connection.

In this first embodiment, a combination of joint preequalization andrake reception is employed.

By means of a decision making device in the transmitter, as a functionof at least one data transmission parameter, a decision is made as towhether an interference treatment of the data signal to be transmittedis to be performed in the transmitter 1 or in the receiver. The datatransmission parameter can for instance be the rate of change of thedata channel, or a measure of it.

If for certain data codes the decision is that the interferencetreatment will be performed in the transmitter 1, then a jointpreequalization takes place. In it, channel estimates for connectionwith interference treatment in the receiver are selected as a Diracpulse function (delta function-pulse response of an ideal channel). Thedetection of the data with pulse function channel estimate is done bymeans of a rake receiving device in the receiver.

FIG. 2 is an illustration of a transmitter device for explaining asecond embodiment of the invention.

In FIG. 2, in addition to the reference numerals already listed, 45designates an extrapolator.

For preequalization of the data signals to be transmitted, the pulseresponse of the current transmission channel in the transmitter 1 mustbe known. However, the channel can be estimated only a certain length oftime before the transmission. The faster the channel changes, the morethe estimate deviates from the current channel. This makes the datatransmission highly erroneous, for instance when there are high relativespeeds between the receiver and the transmitter.

In this second embodiment, the pulse response of the current channel ispredicted from a plurality of prior channel estimates, or extrapolatedlinearly by the extrapolator 45, for instance. This leads to a markedimprovement in the transmission when the channel is changing quickly.

The estimation of the channel pulse responses by the preequalizingtransmitter 1 is done during time segments when this station isreceiving data. The linear extrapolation of the channel pulse responsesestimated during the data reception is applied to the transmission timeperiod. The preequalization of the transmitted signal is done inaccordance with the extrapolated channel estimate.

FIG. 3 shows a receiving device for explaining a third embodiment of theinvention, and FIG. 4 shows an estimation procedure, for the channelestimate used for transmitting signals, performed by the receivingdevice of FIG. 3.

In FIGS. 3 and 4, in addition to the reference numerals already listed,2 is a receiver; 60 is a filter; 70 is a rake receiving device; 80 is achannel estimator; 90 is a device for estimating the channel estimateused in the transmission; D′ stands for received data; SZ is atransmission time segment; EZ is a reception time segment; K, K′ arechannel estimates; M is an averaging operation; KS is a channel estimatefor the transmitting time segment, and t is the time.

The channel pulse response used for the preequalization is estimated inthe receiver 2, in this embodiment. Using this estimate, thepreequalization is taken into account in the receiver, and the data aredetected by interference-treating methods.

In a first example of this embodiment, there is one point formulti-point preequalization with rake reception, as illustrated in FIG.3.

This is a TDD mode, with regularly alternating reception andtransmission time segments EZ/SZ, with code division multiple access(CDMA).

In it, an estimate of the channel pulse response used by the transmitter(the preequalizing station) is made by the receiver (rake station). Thisestimate of the channel is performed during time segments that surroundthe time segment that is used in the preequalizing transmitting stationfor the channel estimate (transmitting time segment of the rakestation=receiving time segment of the preequalizing station). Next,averaging of the two channel estimates is done.

The filter 60 is used on the received signal, using this estimated pulseresponse.

Finally, data detection takes place by means of the rake receivingdevice 70.

In a second example of this embodiment, there is one point formulti-point preequalization with pseudoinverse detection, using theaveraged channel estimate instead of the rake detection.

A discrete-time CDMA transmission system with block transmission isassumed. Let d ^((k))=(d^((k)) ₁, . . . , d^((k)) _(M)), where k=1, . .. , K is the vector of the M data symbols to be transmitted of thek^(th) user. CDMA encoding and preequalization are linear projections ofthe data vectors d ^((k)) onto the signal vectors s ^((k)) to betransmitted. These vectors are added together to make the total signal sand are broadcast by the transmitter:

${\underset{\_}{S}}^{T} = {\sum\limits_{k = 1}^{K}\;{B^{(k)} \cdot {\underset{\_}{d}}^{{(k)}^{T}}}}$in which d ^((k)T) stands for the transposed vector d ^((k)). B^((k)) isthe (M·Q+W−1)×M projection matrix, containing encoding andpreequalization, with the spread factor Q of the CDMA codes and havingthe length W for the channel estimates used in the preequalization.

The signal s is transmitted to the k^(th) user via the k^(th)transmission channel. Let h ^((k))=(h₁ ^((k)), . . . h_(W) ^((k))) bethe pulse response of this channel, and let

${\left. {H^{(k)} = \underset{\underset{{M \cdot Q} + W - 1}{︸}}{\begin{pmatrix}h_{1}^{(k)} & 0 & 0 \\\vdots & \ddots & 0 \\h_{W}^{(k)} & \vdots & h_{1}^{(k)} \\0 & \ddots & \vdots \\0 & 0 & h_{W}^{(k)}\end{pmatrix}}}\mspace{14mu} \right\}{M \cdot Q}} + W - 1 + W - 1$be the corresponding convolution matrix. If there is additive noise n^((k)) of the channel, the k^(th) user then receives the signal

${\underset{\_}{r}}^{{(k)}^{T}} = {{\sum\limits_{l = 1}^{K}\;{H^{(k)} \cdot B^{(1)} \cdot {\underset{\_}{d}}^{{(l)}^{T}}}} + {\underset{\_}{n}}^{(k)}}$

With the reception matrixR^((k))=(B^((k)H)·H^((k)H)·H^((k))·B^((k)))⁻¹·B^((k)H)·H^((k)H), thereceiver obtains from this estimates d^ of the transmitted data inaccordance with the equation{circumflex over (d)} ^((k)) ^(T) =R ^((k)) ·r ^((k)) ^(T)in which H^((k)H) stands for the conjugated complex, transposed matrixH^((k)).

Although the present invention has been described above in terms ofpreferred exemplary embodiments, it is not limited to them but insteadcan be modified in manifold ways.

The invention can be employed wherever signals to be transmitted have tobe preequalized and at the same time connections over quickly changingchannels are required, especially in TDD radio transmission systems withpreequalization.

The invention claimed is:
 1. A data transmission method, comprising thesteps of transmitting an encoded data signal in form of a data stream ofdata bursts between a transmitter and a receiver; making a decision atthe transmitter as a function of at least one data transmissionparameter as to whether an interference treatment of the data signal tobe transmitted will be performed in the transmitter or in the receiverbased on prior channel estimates; in the event that interferencetreatment will be performed at the transmitter performing theinterference treatment on at least one coded channel in the transmitter;and in the event that interference treatment will be performed at thereceiver performing the interference treatment of the data to betransmitted on at least one further coded channel in the receiver; andusing as the at least one data transmission parameter on which thedecision is made the at least one data transmission parameter which is arate of change of an estimate of a pulse response of the data channel.2. A data transmission method as defined in claim 1, wherein the datasignal is CDMA-encoded.
 3. A data transmission method as defined inclaim 1; and further comprising pre-distortion of codes of the datasignal to be transmitted; and using for some codes a Dirac-pulsefunction as a channel estimate.
 4. A data transmission method as definedin claim 3; and further comprising detecting the data with a channelestimate performed as a pulse function, in the receiver by a deviceselected from the group consisting of a rake receiving device and areceiving device.
 5. A data transmission method as defined in claim 3;and further comprising predicting the channel estimate for thepre-equalization from a plurality of prior channel estimates.
 6. A datatransmission method as defined in claim 5; and further comprisingcalculating the channel estimate to be predicted from the prior channelestimates by linear extrapolation.
 7. A data transmission method asdefined in claim 1; and further comprising performing by the receiver anestimate of the channel pulse response used by the transmitter; anddetecting data using this channel estimate.
 8. A data transmissionmethod as defined in claim 7; and further comprising filtering areceived signal with the estimated pulse response; and then detectingthe data by means of a rake receiving device.
 9. A data transmissionmethod as defined in claim 7; and further comprising using the estimatedpulse response for pseudoinverse detection.